1. Field of the Invention
The present invention relates to an electric system for a fuel cell, a fuel cell vehicle, and a method of supplying electric power, and more particularly to an electric system in which a fuel cell and an electric storage device are connected in parallel to each other with respect to a load and connected to the load, and a DC-to-DC converter is connected closer to the electric storage device than the junction where the fuel cell and the electric storage device are connected in parallel to each other, a fuel cell vehicle incorporating such an electric system, and a method of supplying electric power in such an electric system.
2. Description of the Related Art
Recently, fuel cell vehicles carrying fuel cells, which are of excellent fuel efficiency and environment-friendly nature, as propulsive power sources have been developed and put to practical use. Fuel cells for use on fuel cell vehicles are often high-voltage fuel cells because they are required to generate large electric power to provide sufficient drive forces to propel the fuel cell vehicles.
Some fuel cell vehicles also carry electric storage devices for assisting in supplying electric power to meet high loads and load variations and also for storing regenerated electric power.
If the voltage generated by a fuel cell and the rate voltage of an electric storage device are different from each other, then a DC-to-DC converter for performing bidirectional voltage conversion is connected between a power supply line connected to the fuel cell and a power supply line connected to the electric storage device. The DC-to-DC converter allows electric power to be efficiently transferred between the fuel cell and the electric storage device and also allows regenerated electric power to be efficiently stored in the electric storage device.
Fuel cell vehicles incorporate fuel cell accessories for operating fuel cells, i.e., an air compressor, a hydrogen supply pump, a coolant circulation pump, etc., in addition to ordinary motor vehicle accessories including an air conditioner, windshield wipers, power window motors, etc.
One type of connections used for supplying electric power to motor vehicle accessories and fuel cell accessories on fuel cell vehicles is disclosed in Japanese Laid-Open Patent Publication No. 2004-193063. As shown in FIG. 13 of the accompanying drawings, the disclosed system has a power supply line 3 interconnecting a fuel cell 1 and an inverter 2, and power supply lines are branched from the power supply line 3 for supplying electric power to a motor vehicle accessory 4 and a fuel cell accessory 5. The power supply line 3 carries a high voltage suitable for energizing an electric motor. The electric power from the power supply line 3 is reduced in voltage by a DC-to-DC converter 7 before it is supplied to an electric storage device 6. The DC-to-DC converter 7 is capable of converting voltages bidirectionally.
Japanese Laid-Open Patent Publication No. 2002-118981 discloses another system of connections as shown in FIG. 14 of the accompanying drawings. As shown in FIG. 14, a power supply line 8 interconnects an electric storage device 6 and a DC-to-DC converter 7, and power supply lines are branched from the power supply line 8 for supplying electric power to a motor vehicle accessory 4 and a fuel cell accessory 5. The power supply line 8 is of a low voltage because the voltage carried thereby is reduced by the DC-to-DC converter 7 to a value lower than the voltage carried by a power supply line 3 interconnecting a fuel cell 1 and an inverter 2.
If accessories are to be added to the system disclosed in Japanese Laid-Open Patent Publication No. 2002-118981, then they are connected to the low-voltage power supply line 8 that is connected to the electric storage device 6 shown in FIG. 14. However, the system with the added accessories is disadvantageous in that if the DC-to-DC converter fails and the electric power generated by the fuel cell cannot be supplied to the low-voltage power supply line 8, the distance that the fuel cell vehicle is able to travel thereafter will be shortened. Specifically, since the fuel cell needs to be supplied with a reactive gas for its operation, it is necessary to operate an air compressor, pumps, etc. However, as these accessories are connected to the low-voltage power supply line, they are energized by only the electric power which remains in the electric storage device. Consequently, even if a sufficient amount of hydrogen gas is stored in the fuel tank, since no electric power is obtained from the stored hydrogen gas for actuating the air compressor and the pumps, the fuel cell is shut off at the time the discharge of electric power from the electric storage device is finished. In addition, inasmuch as the air compressor and the pumps consume a relatively large amount of electric power if the DC-to-DC converter fails, the electric storage device discharges the stored electric power at a high rate and cannot be operated continuously for a long period of time.
In either one of the systems disclosed in Japanese Laid-Open Patent Publication No. 2004-193063 and Japanese Laid-Open Patent Publication No. 2002-118981, the fuel cell is liable to be deteriorated if the fuel cell vehicle continues to travel in the event of a DC-to-DC converter failure. Specifically, though the amount of electric power generated by the fuel cell is set depending on the amounts of a fuel gas and a reactive gas that are supplied, there are limitations to sudden changes in the supplied amounts of the fuel gas and the reactive gas because of fluid inertia, piping resistances and air compressor dynamic characteristics, possibly causing the system to fail to respond to a required change in the electric power, i.e., to fail to be sufficiently responsive. If the DC-to-DC converter suffers a failure, as no assistive electric current is supplied from the electric storage device, the fuel cell needs to generate electric power in excess of an allowable change in electric power, resulting in insufficient capabilities of the pumps for supplying the fuel gas and the reactive gas. As a consequence, the fuel cell operates under shortage of gas, and is liable to be deteriorated.
Once the fuel cell is deteriorated, it is difficult to restore. Consequently, even when the faulty DC-to-DC converter is repaired, the fuel cell remains deteriorated, and is liable to have its power generating ability lowered and also have its service life reduced.